A very wide range of metal alloys are used for different applications, each alloy offering a particular combination of properties, including strength, ductility, creep resistance, corrosion resistance, fatigue resistance and castability. For example, although pure titanium is highly resistant to corrosion, its corrosion resistance can be improved by forming an alloy with 0.15 wt % palladium. Likewise, Ti-6Al-4V is a popular titanium alloy which displays high strength, creep resistance, fatigue resistance and castability. The corrosion resistance of Ti-6Al-4V may also be similarly improved by the addition of palladium.
The global production of titanium is small in comparison with other metals or alloys and the majority of titanium currently produced is for use in the aerospace industries. Other industries, however, have encountered difficulties in sourcing the material they require and have additionally found it undesirable to maintain a large stock of a range of different titanium alloys as a result of the high price of titanium.
Cermets have been designed so that they display characteristics of both the ceramic and metallic components. In this regard, the ceramic component may contribute a high temperature resistance and hardness, while the metal component can contribute plastic deformation. Cermets have found use in the electronics industry (in the manufacture of resistors and capacitors), ceramic-to-metal joints and seals, as well as in medical applications, such as dentistry.
Powder injection molding (PIM) is a well-known method for producing tailored compositions (see, for example, “Injection Molding of Metals and Ceramics” by Randall M. German and Animesh Bose, MPIF Publishers, 1997 (ISBN No. 1-878-954-61-X), which is herein incorporated by reference in its entirety for all purposes). Generally, PIM involves mixing a powder and a binder to form a feedstock, which is then granulated and injection molded to form a “green” body. The green body is then transformed into a “brown” body by removing the binder. The process of debinding may be thermal, the binder can be removed by solvent extraction, or a combination of both methods. Regardless of the method by which the brown body is generated, the final step of the process involves sintering to produce what is known as a “white” body.
One disadvantage associated with PIM in relation to powders having an affinity for reaction with process gases (such as hydrogen, oxygen or nitrogen) is the need for the maintenance of a high level of purity throughout the fabrication process. Depending upon the metal powder being processed, poor control of process gases and temperature excursions can lead to the formation of undesirable levels of, for example, oxide, nitride or hydride impurities within sintered metal bodies. Using the case of titanium PIM as an example, it is well-known that the formation of titanium oxides, nitrides or hydrides can occur under the temperature conditions used during PIM processing and in the presence of, respectively, oxygen, nitrogen or hydrogen. It has been observed that the presence of interstitial alloying elements can have large effects on the properties of alloys and, as such, are carefully specified within standard alloy compositions (see, for example, “Titanium and Titanium Alloys” in Kirk-Othmer: Encyclopaedia of Chemical Technology, 4th Edition, Vol. 24, pg 186-224, which is hereby incorporated by reference in its entirety for all purposes).
A second disadvantage associated with PIM is that the presence of relatively large amounts of organic material in the green bodies, required as the binder effects efficient and reproducible molding operations, can lead to undesirable levels of carbon-based impurities in the final sintered bodies. The use of unsuitable binder compositions and/or of poor process control during the debinding and sintering stages can result in incomplete removal of the binder material, which can become entrapped within the final, sintered body. In the case of titanium and titanium alloys, for example, the presence of carbon impurities is usually specified at a low level, typically less than 0.1%, to avoid the emergence of a brittle and solid carbide phase at levels greater that 0.2% in the alloy (see, for example, the ASTM International list of titanium alloy standards, which is herein incorporated by reference in its entirety for all purposes).
In addition to the possibility of binder formulations generating carbon-based impurities in the white bodies, the interplay between the selection of a binder formulation and the process conditions for the removal of the binder can cause the formation of further undesirable oxygen-, hydrogen- and nitrogen-based impurities in the final sintered bodies. For example, Tables II and III in “Getting better: big boost for titanium MIM prospects” by S. Froes (in Metal Powder Report Volume 61, Issue 11, December 2006, Pages 20-23, which is hereby incorporated by reference in its entirety for all purposes) respectively list a selection of titanium alloy PIM binder compositions and the properties of the sintered alloys produced using those compositions, primarily on laboratory-scale processes. The majority of debinding processes involve thermal- or solvent-based processes or, on occasion, a combination of both. Whilst the solvent-based processes have been shown to be capable of producing sintered titanium bodies with low impurity levels, volumes of contaminated solvent are produced as waste streams that require subsequent handling and disposal. It is evident from a review of these Tables that achieving sintered alloy components with ASTM standard levels of impurities remains a challenge for many practitioners.
Insofar as thermally-based debinding processes are concerned, it is understood that these types of processes would negate the problems associated with disposal of liquid effluent. However, as Froes comments in the afore-referenced article, even those polymer binders known to readily thermally “unzip” to their starting monomers can still leave undesirable residues in sintered titanium MIM bodies. Depolymerization, or unzipping, tends to occur at temperatures close to those where impurity uptake becomes non-negligible, suggested to be at or above 260° C. for components comprising titanium.
US20080199822 (to BASF) describes an apparatus for the continuous catalytic removal of binder from metallic and/or ceramic shaped bodies produced by powder injection molding. The process involves the use of gaseous nitric acid that reacts with the binder. US20080199822, however, is silent with regard to the reduction of the carbon and/or oxygen content which occurs as a result of binder residues remaining in the brown parts. Nor does US20080199822 appear to describe the maintenance of a good level of purity throughout the PIM process.